Cathode heating element for thermionic tubes



Feb. 18, 1936. A. A. STRAUB 2,031,340

CATHODE HEATING ELEMENT FOR THERMIONIC TUBES Filed March 20, 1930 2313 .1 1 12 .2 Fig.5 12

, INVENTOR ANTHONY A. STRAUB ATTORNEY.

Patented Feb. 18, 1936 PATENT OFFICE CATHODE HEATENG ELEMENT FOR THER HIONIC TUBES Anthony A. Stranb, St

mesne assignments, America, New York, Delaware Application March 20,

'7 Claims.

This invention relates to thermionic tubes and particularly to an improved cathode heating element that greatly reduces the time required to bring the cathode to operating temperature when the tube is initially energized.

Insulating mediums that have heretofore been proposed for adequately insulating the heating element from the cathode embodied tubular elements that were sometimes made in one piece for encasing the filament over its entire length in order to obtain adequate mechanical support for the insulated filament.

The insulating element that is herein proposed provides adequate mechanical support for the filament and also directly exposes substantial portions of the filament to the cathode sleeve, so that it becomes unnecessary to transmit heat from the filament through a substantial body of insulation to the cathode sleeve. This greatly reduces the time interval between the initial energization of the tube and the heating of the cathode to operating temperatures.

Since the present invention is capable of use in several different types of thermionic tubes, the general construction of the heating element will be described, independent of the particular formof thermionic tube in which it is used, it being understood that the invention resides in the particular combination of the heating filament with an insulating medium of the character herein proposed when used in conjunction with a hot cathode sleeve of any particular material. Since the remainder of the tube structure is not necessary to a complete understanding of the invention, the heating element only is illustrated in detail.

The accompanying drawing illustrates an embodiment of the invention for use in any form of hot cathode tube that employs alternating current for heating the filament.

Figure 1 of the drawing is a side elevation of a cathode assembly illustrating an embodiment of the invention as applied to a thermionic tube, the remainder of which is broken away to more clearly show the invention;

Fig. 2 is a side elevation of the cathode sleeve shown prior to its assembly in the tube;

Fig. 3 is a side elevation, with the upper portion shown in section, of the insulating element herein proposed and illustrated separate and apart from the filament and cathode sleeve;

Fig. 4 is an elevational view of the heating filament before assembly; and

Fig. 5 is a view in cross section of the assembled heating element taken on line 5- 5 of Fig. 1

. Marys, Pa., assignor, by to Radio Corporation of N. Y., a corporation of 1930, Serial No. 437,572

to show the cross sectional form of the insulating element intermediate its ends and to also show the relation of the exposed heating element to the cathode sleeve.

Fig. 1 of the accompanying drawing shows the heating element that forms the subject matter of this invention mounted in a thermionic tube which has its remaining elements broken away to more adequately illustrate the invention. The stem I of the tube carries a suitable lead-in wire 2 that carries a cathode sleeve 3 from its upper end. The stem also carries a'pair of lead-in wires 5 and 8 that are respectively connected to the ends of a U-shaped heating filament I that is mounted in the cathode sleeve 3 and insulated therefrom by the insulating and supporting element 8, as particularly shown in Fig. 5. The cathode sleeve and the heating filament are of conventional design and may be made of any suitable material and may be connected to the tube base or other parts of the tube in any desired manner.

The insulating and supporting element 8, shown in detail in Fig. 3 and Fig. 5, is of cylindrical form at its ends 9 and Ill and also at an intermediate portion Il. Suitable spaced longitudinal channels l2 and I3 for receiving the leg portions of the heating filament l are provided. The channels 12 and I3 extend the full length of the insulating support. The outer wall portions of the insulator are cut away, as at I4, I5, l6 and I1, to expose the cathode sleeve 3 to direct radiation from the filament I without interposed insulating material over these zones and without sacrifice of adequate mechanical support for e the heating filament. By forming the insulating support for the filament in the manner described, the legs of the heating filament are adequately spaced and insulated from each other over their entire length by insulating material, and the intermediate and end portions of the filament areencircled by portions 9, l0 and l l of the insulating and supporting element, which rigidly holds the filament in proper spaced relation to the cathode sleeve and also permits direct radiation from the heating filament to the cathode sleeve without sacrifice of mechanical strength of the assembled unit and also without any danger of contact of the heating filament with the cathode sleeve that has heretofore resulted in every attempt known to applicant to obtain direct radiation from the heating filament to the cathode sleeve.

The insulating and supporting element 8 is preferably made of magnesia (MgO), although iii any other suitable material may be used for this purpose. It is a well known fact, however, that the heat insulating properties of such a material and of other known materials for the same purpose is very high. This results in an objectionable delay in heating the cathode sleeve to operating temperatures when such material as magnesia is interposed between the filament and the sleeve over the effective length of the heating filament, thus cutting off direct radiation from the heating filament to the cathode sleeve.

In the operation of a thermionic tube embodying this invention, the time interval for bringing the cathode sleeve to operating temperature has been reduced to substantially one-fourth of the time heretofore required in tubes embodying a hot cathode sleeve that is heated from an insulated filament. This is a very material improvement in the operation of the tube, without any sacrifice in mechanical strength of the-support for the filament and without complicated assembly operations.

It will be obvious to those skilled in the art that the cathode sleeve and the heating filament may be connected to remaining portions of the tube in any desired manner without departing from the spirit and scope of the invention herein described.

What I claim is:

1. In a thermionic tube, the combination with a cathode sleeve, and a heating filament of U- shape having parallel leg portions mounted within said sleeve, of an insulating and supporting element comp-rising a. portion interposed between the legs of. the filament over their operating length, and other portions integral therewith and interposed between the filament and the sleeve at each end of the element and at an intermediate zone and, completely surrounding each leg of the filament, the remainder of theinsulating and supporting element being of substantially the same lateral dimension as the spacing of the leg portions of the filament in planes parallel with. the filament, whereby material portions of the sleeve are subjected to direct radiation from the filament.

2. A heating element for thermionic tubes embodying a cathode sleeve, comprising a heating filament ofv U-shape having parallel leg portions mounted within the cathode sleeve, and a unitary insulating support for the filament embodying a filament leg spacing portion extending the full length. of the insulating support and filament spacing means at the ends and at an intermediate portion only of. the. insulating support, completely surrounding and spacing each leg of the filament from the cathode sleeve, whereby substantial. portions of the sleeve are exposed to direct radiation from the heating filament.

3. A heating element for thermionic tubes, comprising a cathode sleeve, an insulating and supporting element disposed within said sleeve of substantially the length of the sleeve and having portions at the ends and an intermediate portion to engage said sleeve, said element having apertures therethrough, a heating filament disposed within said apertures and extending throughout the length of said element, said filament being exposed between the end portions and intermediate portions of the insulating element to radiate heat directly to said sleeve.

4. A heater element for electron discharge devices comprising an insulator member having a central body portion, a plurality of perforated lateral extensions from the body portion, and a heater wire passing through said extensions.

5. An insulator of the class described comprising a body of insulating material having spacedapart enlarged portions thereon with fiat-sided portions intermediate the enlarged portions of less cross-sectional area than the area of the enlarged portions, said body having openings therein passing throughthe enlarged portions thereof through which a heater wire may be passed, the openings being in line and being on opposite sides of the fiat portion.

6. A heating element for a tubular cathode sleeve for thermionic tubes comprising a looped heating filament having leg portionsdisposed side byside within the cathode sleeve, and a unitary insulator having spaced along its length cylindrical portions which fit into said cathode sleeve and smaller intermediate portions of a combined length. greater than the combined width of said cylindrical portions, said cylindrical portions havingperforations between their edges and the surface of said intermediate portions to receive the legs of said filament and hold them in position between said cathode sleeve and the surface of said intermediate portions of said insulator to permit direct radiation of heat to the cathode sleeve from the major portion of the length of said filament.

7..A heating element for a tubular cathode sleeve of thermionic tubes comprising a looped heating filament disposed within the cathode sleeve, and a unitary insulator within and coaxial with said sleeve and having a core portion smaller than said sleeve with cylindrical transverse spacing portions at intervals along said core and contacting with the inside of said sleeve to II alongside said core and directly exposed to theinterior of said sleeve.

ANTHONY A. STRAUB. 

